1. Field of the Invention
The present invention relates to a process for producing a membrane/electrode assembly for polymer electrolyte fuel cells.
2. Discussion of Background
A polymer electrolyte fuel cell is constituted by a plurality of membrane/electrode assemblies stacked one on another via an electrically conductive separator having gas channels formed therein, wherein each membrane/electrode assembly comprises a polymer electrolyte membrane and electrodes (a cathode (an air electrode) and an anode (a fuel electrode)) provided on both sides of the polymer electrolyte membrane. Each electrode is constituted by a catalyst layer being in contact with the polymer electrolyte membrane and a porous gas diffusion layer disposed on the exterior side of the catalyst layer. The gas diffusion layer performs a function to diffuse air or a fuel in the electrode and a function to discharge water formed in the electrode.
The following processes are known as processes for producing membrane/electrode assemblies.
(1) A process wherein on the surface of a gas diffusion layer, a coating fluid containing an electrode catalyst is applied to form a catalyst layer thereby to obtain an electrode, and two such electrodes are subjected to hot pressing for bonding in such a state that a polymer electrolyte membrane is sandwiched between the two electrodes (Patent Document 1).
(2) A process wherein on a substrate film, a coating fluid containing a catalyst is applied to form a first catalyst layer; on the first catalyst layer, a coating fluid containing an ion exchange resin is applied to form a polymer electrolyte membrane; on the polymer electrolyte membrane, a coating fluid containing a catalyst is applied to form a second catalyst layer; from a laminate comprising the first catalyst layer, the polymer electrolyte membrane and the second catalyst layer formed on the substrate film, the substrate film is peeled off; and the laminate and two gas diffusion layers are subjected to hot pressing for bonding in such a state that the laminate is sandwiched between the two gas diffusion layers (Patent Document 2).
However, in the case of the membrane/electrode assembly obtained in the process (1), the catalyst layers of both electrodes are formed by coating directly on the surface of the gas diffusion layers, whereby part of the catalyst layers is likely to penetrate into both gas diffusion layers to clog part of voids in the gas diffusion layers. As a result, the gas diffusion properties of the gas diffusion layers tend to deteriorate, and there will be a problem such that in a high current density region, the output voltage of the polymer electrolyte fuel cell tends to be inadequate.
In the case of the membrane/electrode assembly obtained by the process (2), the polymer electrolyte membrane is formed by coating directly on the surface of the first catalyst layer, whereby part of the ion exchange resin is likely to penetrate into the first catalyst layer to clog many voids in the first catalyst layer. As a result, the gas diffusion properties of the first catalyst layer tend to deteriorate, and there will be a problem such that in a high current density region, the output voltage of the polymer electrolyte fuel cell employing such membrane/electrode assemblies tends to be inadequate.
Patent Document 1: JP-A-04-162365
Patent Document 2: WO02/005371